The present invention relates to means for precipitating charged particles from a gas and more particularly to a novel contamination control device for use in a coaxial gas-insulated electrical bus.
A gas-insulated transmission bus consists of a cylindrical center conductor coaxially mounted within a grounded cylindrical outer shell. The center conductor is conventionally supported by insulating spacers located at discrete intervals along the axis of the bus. The bus may conventionally be insulated with SF.sub.6 gas at pressures ranging between zero psig and 50 psig. As the gas pressure is increased, the dielectric strength and the ultimate expected withstand voltage both increase.
A major limitation on gas-insulated systems occurs by contamination of the insulating space by conducting particles. The particles may be deposited upon interior surfaces during manufacture, by residue overlooked during cleaning of the bus elements or may be generated during assembly of the bus. The dielectric withstanding voltage of a gas-insulated system is greatly affected by particle contamination, such that even though the gas pressure is increased to improve the dielectric strength of the gas, the particles enclosed in the bus will prevent an overall improvement in dielectric properties. Consequently, there is a great need to at least control, if not completely eliminate, the presence of conducting impurities within the gas-insulated volume of the transmission bus.
A number of devices have been proposed to achieve this control of conducting particles. These devices generally provide an area of low electrical stress into which the conducting particles are moved by an electric field and in which area the charge particles cannot acquire sufficient energy to reenter the external, relatively higher electric field areas. Examples of such devices may be found in U.S. Pat. No. 3,515,939 to Trump, wherein a low field region is provided adjacent to the interior surface of a grounded outer shell and in communication with apertures allowing a charged particle to be radially moved into the low-field region; and U.S. Pat. No. 3,813,475 to Cronin, wherein an annular ring is coaxially arranged adjacent to the interior surface of the grounded outer shell to provide an annular volume into which axially moving particles may be received and retained.
In addition to requiring relatively massive components, the devices of both cited references are generally of low efficiency, as the charged particles exhibit a random motion and, even when finally positioned adjacent to the entrance openings of the known devices, tend to collect at the entrance to the low-field areas without full migration into such areas. A quantity of particles thus builds up at the entrance to the low field region and these particles frequently acquire enough energy to return to the high-field region adjacent to the center conductor and decrease the dielectric strength of the insulating gas.